Bonding to Existing Concrete

posted by Bob Monday, November 22, 2010 at 12:00 PM

Fact: Fresh wet concrete does not normally bond well to existing dry concrete. Do you remember elementary school where one of the subjects on which you were graded was “plays well with others”? Concrete would have gotten an F. There is nothing in basic portland cement that will act as a bonding agent. Portland cement concrete works well in mass and provides great compressive strength but not bond.

Concrete is marvelous stuff but in time it will deteriorate. When it does, you either have to patch it or replace it. Assuming that it is structurally sound the least expensive alternative is to patch it. However patching it requires some attention to detail or your patch will not last. So that you don’t waste too much time or money, we should probably discuss what “structurally sound” means. If your sidewalk has either heaved or dropped at almost every joint, repairing it will not provide a long-term solution. The slabs are likely still moving. If your slab has so much sand and gravel on the surface that despite sweeping and sweeping and squirting and squirting it just keeps coming back, don’t waste your time on repairs. If you have multiple cracks that run so deep that they appear to run through the slab, a repair would only be temporary. The solution to all of these problems involves a jackhammer and bags of one of the Sakrete concretes.

Since this discussion is on the best way to bond concrete, we will assume that your slab is good.

There are a variety of Sakrete concrete repair products available to fix concrete that has begun to deteriorate. However, without good surface preparation, none of them are going to perform satisfactorily. All loose sand, gravel, dirt, leaves etc. must be removed. This can typically be done with a garden hose and a good nozzle. Tough areas may require a pressure washer or mechanical abrasion. The two toughest areas to cover are those with oil and tree sap. Both of these will work their way down into the concrete. Simply washing the surface isn’t sufficient. If the stains do not run too deep, you can chip away the concrete using a hammer and chisel. Don’t forget the goggles (not just glasses) as this process will throw concrete all over the place. Also, keep your thumb out of the way. If the spots are too large or too deep for this to be practical, you may need a sealer to cover the stains before patching.

There are two basic methods for bonding a portland cement based product to existing concrete; 1) chemically and 2) mechanically.

Let’s discuss the mechanical approach first since it is really used in both approaches. The most effective way to ensure a really good bond is with a scratch coat. This is simply a very wet coat made up by mixing the repair product with water. Mix up a small amount of the repair material to a soupy consistency. You don’t need to measure the water-just turn the stuff into slop. Then, using a gloved hand or a rag, smear the material onto the area to be patched. Just think finger painting from kindergarten. The technique is about the same. Apply pressure to ensure that as much as possible is shoved into the nocks and crannies. You only need a thin coat. It is not necessary for this scratch coat to dry. By the time you get the repair material mixed it will be ready. Then mix up additional repair material to the proper consistency and apply over this thin scratch coat.

The chemical approach involved mixing up a liquid bonding agent that helps bond new concrete products to old. Products like Sakrete Top'n Bond and Sakrete Flo-Coat already contain polymers that greatly improve the bond of portland cement and should NEVER be used with a liquid bonding agent. I know in America bigger is better but it’s just not so with these products. Other products like Sakrete Sand Mix and Sakrete Fast Set Cement Patcher benefit from the use of a liquid chemical bonding agent such as Sakrete Bonder/Fortifier. When using a liquid bonding agent, paint the bonder onto the existing concrete and allow it to dry until it is tacky. This usually takes only a few minutes. Then apply the repair material. Just as in the process described above, after the bonder has become tacky apply a scratch coat and then apply the repair material. The most effective way to ensure that the bonding agent gets into the existing concrete is to apply it directly using a brush or rag. It can be sprayed if you happen to have a sprayer. Although the directions say that you can use it as part of the mix water, direct application works better.

If you are doing a large area and a scratch coat isn’t practical you will need to spray the surface with water before you apply the repair material. On a warm day, the existing concrete surface will be hot enough to suck the water out of the repair material. In addition, some concretes are quite porous and will rob water from your repair material. If too much water is lost into the old concrete there will not be enough water to hydrate all of the cement particles and a lower strength material will be the result. Concrete simply will not bond to all substances. Paint, oil, glue from old flooring tiles are just a few. You must mechanically remove these materials if you want the job to last.

Once the job is complete, you can do a quick check to see if the bond was successful. Wait at least 24 hours and then tap “gently” on the patch using a hammer or some other dull object and listen for a hollow echoing sound. If you just get a dull thud then the material has bonded well. If you get a hollow sound, the material has not bonded and will crack in time. Which means it is back to the beginning of today’s topic. Here is hoping your concrete work comes across as a dull thud (not like some of my party guests) rather than a hollow endeavor.



Joni, yes the Flo-Coat can be used. The smooth areas will need to be abraded with a grinder or tool that will produce a course profile. If you try to apply over a really smooth towel finish it may delaminate. After abrading the surface, pressure wash the entire area to remove any loose dust or debris. Make sure that the surface is damp before you apply the material. The Flo-Coat's longevity will be no different that the existing concrete if applied properly. Sealing the surface after 28 days with a concrete sealer will help your material last longer. The key to a good repair or application is your prep work. It must be a rough not smooth surface, solid, clean, and damp to achieve a good bond. Yes, the Flo-Coat can be pigmented.
- Lee-Technical Service
Friday, June 26, 2015 at 11:32 AM

My old walkway(20+yr)in cold rainy Oregon is approx 8x25 and porch 8x6,the long stretch currently has 2 cracks crosswise, which don't bother me but no other expansion joints. Some of it is roughed from weather and some of it is still very smoothe under the eaves. What is the longevity of a resurfacer like Flo Coat Resurfacer? What kind of maintenence will be required? Does it need a sealer after it is applied, if so how soon? I'm afraid of peeling/cracking even with a good initial bond. Can the product be tinted before application?
- Joni
Thursday, June 25, 2015 at 1:53 AM

RT, the concrete should slope so that water drains away and does not pool on the surface. We suggest using shims for the hot tub if it does not already have them built in.
- Chris Technical Services
Friday, June 12, 2015 at 9:31 AM

I want to level my concrete pad so I can put a hot tub on. The problem is the current pad slopes 2" in the 8'. What is the best way to level it and make sure the cap doesn't slide with the weight?
- RT
Thursday, June 4, 2015 at 9:43 PM

Sandman, Portland cement by itself has no strength unless mixed with an aggregate and has no ability to be placed at a thickness other than as a neat application in its raw state. The existing poured concrete for your table top will need to cure for 28 days before you can apply a topping material. The Sakrete Top ‘N Bond or Sakrete Flo-Coat can be used for resurfacing your concrete tabletop and can be applied from ½” down to a feather edge. When using a concrete stain make sure to check with the manufacture for their recommendations on when to apply the stain over freshly placed concrete. If you have any other questions please do not hesitate to contact us.
- Chris Technical Services
Thursday, May 28, 2015 at 1:02 PM

Hi. I am building a deck that requires the use of concrete piers to mount the 6x6 posts. The piers are 36" deep and 14" wide. I was told to fill the piers with concrete and leave 12" at the top. Then once it cures to go back and fill the rest of the pier up and add the adjustable post bracket. Does this sound right? Will the second batch of concrete bond to the first batch that has already cured? Any help would be appreciated, thanks!
- AC
Thursday, May 28, 2015 at 11:15 AM

Created a pour in place outdoor table. Due to lack to material at the end of the work, the top was uneven. So a few days later I made a pure Portland cement slurry and sort of trowelled on top of it. Looked like it was ok but after two day it cracked and peel off. Thickness was at the most quarter inch to slightly thicker. Being my first time, I scraped off the whole top and then again tried it with thickness of about half inch. Again after three days it cracked up and now in the process of scraping, chiseling off the top wasted about a bag of Portland ( its a learning process). To Sakrete engineers or some one who knows stuff about concrete , Can I use your FLO-COAT to even the top of my concrete table? Yes, I will be staining it later. Please, I would really appreciate some clues
- sandman
Wednesday, May 27, 2015 at 11:07 AM

Cheri, Sakrete Flo-Coat is perfect for this type of repair. First be sure to remove any loose material, dirt, dust, paint, stain, etc. Sakrete Flo-Coat can be applied from ½” down to as thin as a feather edge. To apply Sakrete Flo-Coat use a squeegee and pull the material into place. Once in place finish by adding texture with a soft bristle broom by lightly pulling the broom across the surface.
- Chris Technical Services
Wednesday, May 20, 2015 at 9:18 AM

Dean, yes, you can add another layer of Sakrete Flo-Coat once it has cured for at least 28 days.
- Chris Technical Services
Wednesday, May 20, 2015 at 9:09 AM

I have a stamped concrete patio that, apparently was never sealed. I made a rookie mistake this winter and threw salt down on it the melt some thick ice that had formed and when the ice flow melted I had ruined my patio! The top layer of the concrete bubbled and peeled off! It is a fairly large patio and also fairly deep. There is road base/old patio under it and it is about three and half feet deep! Clearly, it would be cost prohibitive to jackhammer out and replace. What would be the best method for me to use to fix this?
- Cheri
Tuesday, May 19, 2015 at 11:01 PM



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